Thermographic recording processes and materials



May 26, 1970 L. M. DE HAL-:s ET AL 3,514,597

THERMOGRAPHIC RECORDING PROCESSES AND MATERIALS Filed April 5, 1967 vINVENTOR 5 oow.) man@ u NNN.,

ATTORNEYS United States Patent O U.s. C1. 25o- 6s 16 Claims ABSTRACT FTHE DISCLOSURE A heat-sensitive recording material comprising a snpport,a Water-permeable hydrophilic colloid layer adjacent said support and anexternal water-impermeable hydrophobie layer in contact with saidcolloid layer are subjected to an image-wise action of heat, so that adecrease of hydrophobicity is effected in the heat-exposed areas of saidexternal layer.

The present invention relates to thermographic recording `and materialssuited therefor.

From the U.S. patent application Ser. No. 421,861 filed Dec. 29, 1964',it is known to record a heat pattern as a diiferentiation ofhydrophility and water-permeability in a recording material containing alayer comprising hydrophobic thermoplastic polymer particles, which aredispersed in a hydrophilic binder. The heat-exposed parts of saidhydrophilic layer become less water-permeable and less hydrophilic thanthe unexposed parts so that by treating the exposed recording layer withan aqueous dyestuff solution, a dye-image is obtained.

The image obtained is a legible negative if the heatsensitive materialis exposed to heat in correspondence with the image-markings of apositive original.

The present invention provides recording materials with methods thatpermit the direct production of positive records from positiveoriginals.

In accordance With the invention, a recording material comprising aWater-impermeable hydrophobic surface layer containing (a) hydrophobicmeltable substance(s), wherein said surface layer is in contact with ahydrophilic interlayer that in its turn is supported directly orindirectly (i.e. through one or more intervening layers) by a base e.g.a sheet of paper, is subjected to an image-Wise or record-Wise action ofheat, so that a decrease of hydrophobicity is effected in theheat-exposed areas of the surface layer.

In other Words, as a result of the modulated heating the recording layercomprising said meltable substance bears a record of the heating patternin terms of a difference in hydrophobicity and water-permeability.Depending on the composition of the recording material a visible imagecan be formed by a subsequent development step, e.g., application of anaqueous dyestuff solution, which becomes selectively or differentiallyabsorbed.

The methods and materials according to the invention are intendedprimarily for document copying and like purpose, but the invention isnevertheless capable of `wider application, e.g., for recording dataapplied as a heat pattern to the recording material responsive totransmitted signals.

f The application of the heat pattern may be achieved by heating theimage markings of an original while these are in heat-conductiverelation with the recording material. Such heating may, e.g., beperformed by exposing the original to infrared radiation, the imagemarkings being capable of absorbing such radiation, or by exposing the"ice original to visible light of suicient intensity, if theimagemarkings are light-absorbing, the absorbed light being transformedinto heat.

One way in which reproductions of matter such as typewritten documentsmay be performed is by placing the copying sheet with its fusiblesurface layer against the back or unprinted surface of the document, andthen exposing the printed face of the document to infrared radiation.Absorption of radiation by the printed characters results in generationof heat, which is then conducted through the backing e.g. to theheat-sensitive surface, thus producing the desired change inhydrophility therein. This back-printing method is capable of producingdirect reproductions if the original is printed on a satisfactorily thinand heat-conductive paper or other material. This method is not suited,however, for copying from heavy book paper or thin papers heavilyprinted on both sides.

In front-printing the effective radiation rst passes through theheat-sensitive copying sheet, which consequently should be sufficientlypermeable to the radiation used.

The heat pattern resulting from absorption of radiation at the imagemarkings of the document causes the necessary image-Wise heating of therecording material. According to whether the image markings make contactwith the heat-sensitive surface layer during the exposure or stand incontact with the support, i.e. a very thin heat-conductive support, animage, which is legible through the support (transparent support) or animage, which is legible by reflected light, is obtained.

Selective heating of a recording material of use in the invention canalso be realized by image-wise exposing a recording 'material whichcontains infra-red and/ or visible light-absorbing substance(s)converting absorbed light into heat to infra-red land/or visible light.Said substances can be incorporated in the recording layer itself (thelayer comprising the fusible substance), and/or in the adjacenthydrophilic interlayer and/or in the paper or other support. If suchradiation-absorbing substance is not incorporated into the recordinglayer itself, it must nevertheless be in heat-conductive relationshipwith such layer.

Other heat-sensitive recording materials, which materials compriselightand/or infrared-absorbing substances that generate heat in thematerial to bring about some modification of the properties of aheat-sensitive layer when the material is exposed to suitable radiation,are useful in connection with the present invention. Some specificexamples of substances that absorb visible light and infra-redradiation, and which may be incorporated into recording materialsaccording to the invention, are i.e. finely divided carbon particles,e.g. carbon black, and heavy metals in finely divided state e.g. silver,bismuth, lead, iron, cobalt, and nickel. The particle size of thesesubstances preferably does not exceed 0.1M.

In recording an original according to the invention, the recordingmaterial incorporating light and/or heat absorbing substance(s), ispreferably subjected to a short duration high intensity exposure. Duringsuch exposure the image markings may be in heat-conductive relationshipwith the hydrophobic surface layer. The exposure time preferably doesnot exceed 10-1 seconds. In such very short exposure there isinsufficient time for any heat generated in the image markings to besufficiently conducted to the said surface layer to bring about aneifective change therein; on the other hand the light-absorbingsubstance(s) in the surface layer are heated and thus provide thenecessary heat for melting or softening that layer.

The short duration exposure may lbe performed by using a flash lampsupplying a light energy of to 1000 watt sec. in a time interval of 10-5to 10-2 seconds.

For obtaining a suicient image differentiation an energy of at least 0.3watt sec./cm.2 is required.

Of course, gas discharge lamps with a much lower energy output can beused if the emitted energy is focussed onto a relatively small area ofthe recording material. Obviously the exposure may be carried outprogressively or intermittently, e.g., it may be a scanningwise exposureby means of Ia high-intensity light spot image-wise modulated andrapidly line-wise scanning the heat-sensitive surface layer, or it mayIbe a progressive exposure through a slot wherein, e.g., light of atube-like radiation source is focussed.

On exposure, the hydrophilic interlayer normally acts las a receivinglayer for the molten substance(s) of the hydrophobic layer. Thehydrophilic layer and the hydrophobic layer can be composed in such away that substances thereof become compatible by heating (melting).

The hydrophobic layer is preferably composed of or mainly contains(preferably more than 80% by weight) a hydrophobic substance or amixture of such substances having preferably a melting point comprisedbetween 40 and 200 C. and a low melt viscosity.

Substances that are preferred in this respect are waxlike hydrophobicorganic compounds preferably containing higher aliphatic groups with eg.to 20 carbon Iatoms more particularly fatty acids and the metal saltsthereof, fatty alcohols and ethers, fatty acid esters, sterols, fattyketones, and parainic compounds.

Thus, waxes of the known six classes i.e. vegetable, mineral, insect,petroleum, animal, and synthetic waxes can be used.

Carnauba wax, ouricury wax, candelilla wax, and sugar cane Wax belong tothe vegetable wax class. Ozokerite, montan wax, and Utah wax are mineralwaxes. Beeswax, shellac wax, and Chinese insect wax belong to the insectclass. Paraiiin wax is a -member of the petroleum class. Woolwax andspermacetic wax from the sperm whale is in the animal class. Chemicallymodified natural waxes such as the IG ywaxes made from the naturalmontan wax can also be used. Another modified natural wax is a partlyoxidized paraffin that can be a substitute for carnauba wax. Long chainpolymers of ethylene e.g. Epolene N marketed by Eastman ChemicalProducts, Inc., Kingsport, Tenn., U.S.A., halogenated hydrocarbons, e.g.chlorinated or fluorinated paran and silicone waxes belond to thesynthetic class.

All these waxes can be mixed with each other and/or to some extent withoil. Among the above-mentioned waxes carnauba wax has the best Howcharacteristics and the best oil retention properties. Vegetable,insect, and animal waxes are usually composed of a mixture of varioushigh-melting fatty acids, alcohols, and esters.-

For more details about waxes and wax-like hydrophobic thermoplasticmaterials reference can be made to The Chemistry and Technology of Waxesby A. H. Warth, 2nd ed. 1956, Reinhold Publishing Corporation, New York,U.S.A.

Suitable hydrophobic organic compounds having a wax-like consistency anda melting point comprised between 40 and 200 C. are water-insolublearomatic compounds e.g. phthalic acid anhydride, biphenyl, solid terpenecompounds, and the derivatives thereof such as camphene, camphane, andcamphor. Higher aliphatic aldehydes, ketones, ethers,nitrogen-containing hydrophobic organic compounds such as amides,anilides, sulphur-containing compounds e.g. biphenyl sulphone andthionaphthene quinone. Halogenated organic compounds such asa-bromocamphor, carbon tetrabromjde, trichlorobenzene, hexachloroethane,and iodoform.

The hydrophilic interlayers preferably contain or are composed ofhydrophilic hardened or unhardened organic hydrophilic colloids e.g.:

(A) Water-soluble natural polymeric substances e.g. agar-agar,alginates, arnylose, amylopectin, dextrin, gum arabic, casein, andgelatin;

(B) Mo'died natural polymeric substances derived from starch, cellulose,and alginic acid.

Suitable water-soluble starch derivatives eg.:

(l) Anionic starches-starch acetate, e.g. corn acetate containing l to4% of acetyl groups; starch hydroxyethyl ethers; hydroxyalkylstarchderivatives (eg. hydroxyethyland hydroxypropylstarch derivatives).

(2) Ionic starches (the introduction of carboxyl, sulphonate, sulphate,or phosphate groups into starch permits the preparation of water-solublealkali metal or ammonium salts of anionic starches, whereas theintroduction of amino groups permits the preparation of cationicstarches e.g. tertiary aminoalkyl corn starch).

Suitable water-soluble cellulose derivatives are e.g.:

(l) Non-ionic cellulose derivatives (this category includesmethylcellulose, hydroxyalkylcellulose derivatives andpolyoxyalkylenecellulose derivatives).

(2) Ionic cellulose derivatives (the introduction of carboxy,sulphonate, sulphate, or phosphate groups into cellulose permits thepreparation of water-soluble alkali metal or ammonium salts of anioniccellulose derivatives).

Suitable anionic cellulose gums are e.g.: carboxymethylcellulose,methylcellulose-m-sulphobenzoate, ethylcellulose sulphosuccinate andacetylcellulose sulphosuccinate.

(C) Completely synthetic water-soluble resins.

Among these resins there can be mentioned:

(l) fWater-soluble homoand co-vinyl polymers containing e. g. vinylalcohol, N-vinyl-pyrrolidinone, vinyl methyl ether, acrylic acid,methacrylic acid, maleic acid, acrylamide, or styrene sulphonic acidunits.

(2) Water-soluble homoand copolymers containing alkylene oxide unitssuch as ethylene oxide units.

(3) Water-soluble polycondensates e.g. water-soluble alkyd resins andwater-soluble polyester compounds.

(D) Hydrophilic water-insoluble but water-permeable polymers, e.g.ethylcellulose.

Among the above-mentioned polymers particularly suited ones are thosewhich on heating become compatible with the molten hydrophobicsubstances of the hydrophobic top layer. The hydrophilic interlayer maycontain in addition to the hydrophilic polymer or mixture of polymerssubstances with polar and non-polar groups in the same molecule, e.g.wetting or emulsifying agents.

Examples of suitable ionic emulsifying and wetting agents are: thesodium salt of oleyltauride, sodium stearate, heptadecenyl-benzimidazolesulphonic acid sodium salt, sodium sulphonates of higher aliphaticalcohols e.g. Z-methylhexanol sodium sulphonate, sodiumdi-iso-octylsulphosuccinate, sodium dodecyl sulphate and tetradecylbenzene sulphonic acid sodium salt. Examples of suitable non-ionicemulsifying agents are: saponine, condensation products of alkyl phenolsand ethylene oxide, e.g. octylphenyl polyglycol ether,isononylphenoxypoly(ethylenoxy)ethanol and polyethylene glycol oleate.

A survey of emulsifying agents and wetting agents, representatives ofwhich can be applied according to the present invention, is given byGerhard Gawalek Waschund Netzmittel, Akademieverlag, Berlin (1962).

The hydrophilic interlayer may contain all kinds of image-formingsubstances, e.g. the hydrophilic interlayer may contain light-sensitivesilver halide, developing substances therefor, `developing nuclei forcomplexed silver halide, catalysts for colour reactions, dyes, which aresoluble in water and/or organic solvents, and pigments, metal particles,which can be etched away and fillers fwith a substantially hydrophiliccharacter e.g. barium sulphate, zinc oxide, titanium dioxide, andsilica.

The support of the recording material may be porous or non-porous. Forinstance it may be composed of fibers such as paper, or it has ascreen-like structure such as a nylon gauze or it can be a non-porousmaterial such as glass, glazed porcelain, a metal foil, a natural orsynthetic resin film, a resin impregnated porous material, or a supportcoated with a metal film, foil, or hydrophobic resin layer.

When applied `to a porous support the hydrophilic interlayer preventsthe hyrophobic substances from penetrating intoy the support.

A particularly suitable base material is glassine paper, which is ahydrophilic transparent paper base material and which is preferablycoated with a fat-absorbing hydrophilic interlayer containingemulsifying or wetting agents.

Suitable fat-permeable paper supports are coated papers (papier couch)e.g. art printing paper.

The hydrophilic interlayer, if any, preferably has a thickness notgreater than p.. The hydrophobic meltable layer preferably has athickness varying between 2 and u.

The hydrophobic meltable recording layer is preferably applied in theform of a solution in an organic solvent that does not dissolve thehydrophilic interlayer, eg. a solvent that is not miscible Iwith water.After coating the hydrophobic layer must possess a tight structure, i.e.it should be water-impermeable. Depending on the type of hydrophobicsubstance the hydrophobic layer can also be applied advantageously bysublimation e.g. in vacuum or under atmospheric conditions at elevatedtemperature. Sublimable substances, which can be applied in that way,are e.g. phthalic anhydride, biphenyl, and camphor.

As a result of the image-wise or record-wise heating, the hydrophobicsubstances in the hydrophobic layer melt in correspondence with the heatpattern. If the underlying hydrophilic layer contains polymerichydrophilic substances that become compatible with the moltenhydrophobic substances, said hydrophobic substances diffuse into thehydrophilic layer in other words they fuse homogeneously with thehydrophilic substances.

In order to determine, whether a hydrophobic substance is suited or notto become compatible, while in fused state, with a given hydrophilicpolymer, a method can be applied, which consists in determining themelting point of the hydrophobic fusible material alone and of themixture of said fusible material with the polymer chosen. Prior to thedetermination of the melting point the mixture must have been heatedbeyond the melting point of the hydrophobic substance and then cooled.In this test a substantial increase in the melting point of the saidmixture beyond that of the fusible material alone indicates that thesubstances are compatible, whereas a substantially unchanged meltingpoint indicates a lack of compatibility. In this type of test thehydrophilic polymer alone has a melting or softening point higher thanthat of the hydrophobic fusible substance.

The heat-exposed material can be developed e.g. by applying to theimage-wise or record-wise heat-treated hydrophobic layer a hydrophilicsolution e.g. an aqueous dyestutf solution that penetrates into therecording material in correspondence with the heated areas.

When penetrating into the heated areas that are renderedwater-permeable, the hydrophilic solution, 'which can be considered tobe a developing solution, can according to a special embodiment formtherein a dyestuf image upon reaction with colour-forming components.The developing solution according to the latter embodiment may contain adiazonium coupling agent whereas the recording material may comprise asuitable coupling partner therefor. The developed element can now beused as such, or as a master for making multiple copies by printing. Ifthe dyestuff has a water-soluble character, the obtained copy can beused as a hydrotype printing master. If the dyestuit absorbed in therecording material is soluble in a lower aliphatic alcohol, the obtainedmaster can be used for hectographic printing. In the latter case thehydrophobic layer is obviously insoluble in the alcohol used for theprinting.

In this connection it is interesting to note that according to aparticularly interesting embodiment a hectographic printing master canbe obtained by simple imagewise heating of a recording materialaccording to the present invention, which in the hydrophilic interlayeror a layer in water-permeable relationship therewith contains adyestuff, which is suited for hydrotype or hectographic printing. Dyessuited for hectographic printing have a hydrophilic character, such asethanolor watersoluble dyes e.g. triphenylmethane dye salts.

According to another special embodiment a recording material is usedcontaining in the porous and/or hydrophilic element substances that canbe bleached or etched by the action of substances present in ahydrophilic developing liquid.

In this respect reference is made eg. to colloidal silver, which can bebleached by a common photographic bleaching bath preferably containingpotassium hexacyanoferrate (III). Use can also be made of oxonol dyes,which as generally known are used in dischargeable filter andantihalation dye layers in common silver halide photography and can bebleached by a solution having reducing properties e.g. a photographicsilver halide developing bath.

The following examples illustrate the present invention.

EXAMPLE 1 Onto a porous paper web a hydrophilic interlayer was coated ata rate of 20 sq. m. per litre from the following composition:

Carboxymethylcellulose, g. 20 12.5% aqueous solution of saponine, cc.l() Water, cc. 970

After having been dried this interlayer was coated twice at a rate of 20sq. m. per litre with the following hydrophobic composition:

Carbon tetrachloride, cc. 975 Behenic acid, g. 25

The material was then drie-d at room temperature.

EXAMPLE 2 On a translucent glassine paper of 60 g./sq. m. a hydrophilicinterlayer of the following composition:

Hydroxyethylstarch, g. 20 12.5% aqueous solution of saponine, cc. 10Water, cc. 970

was coated at a rate of 20 sq. m. per litre.

To the dried interlayer a hydrophobic layer of the followingcomposition:

Carbon tetrachloride, cc. 950 Beeswax, g. 50

was applied at a rate of 20 sq. m. per litre. After having been coatedthe material was dried at room temperature.

EXAMPLE 3 A cellulose triacetate support provided with a gelatin subbinglayer was coated at a rate of 20 sq. m. per litre with a hydrophilicinterlayer from the following composition:

Gelatin, g. 50 12.5% aqueous solution of saponine, cc. 10 Water, cc. 940

The layer was dried at room temperature.

7 EXAMPLE 4 A hydrophobic coating consisting of biphenyl was applied bysublimation to a starch-coated paper weighing 90 g. per sq. m. Thesublimation was carried out by heating the biphenyl to 70 C.

The resulting material was processed in the same Way as the materials ofExample 1 according to the following procedure:

The image markings of an original presenting a carbonink printed text ona paper base were brought in contact with the support of the recordingmaterial. The recording material in contact with the original wasreectographically exposed to infrared radiation by means of an Ormig(trade mark) thermographic copying apparatus. The hydrophobic layer sideof the exposed recording material was then brought in contact with anaqueous dye solution containing 2% by weight of a 50:50 mixture ofmalachite green and fuchsin.

In this way a legible positive copy was obtained from the original.

The materials described in the Examples 2 and 3 were exposed toinfra-red light in the same way as described above, with the provisiohowever, that during the exposure the hydrophobic layer was placed incontact with the image markings of the original. After application ofthe above-described aqueous dye solution to the hydrophobic layer a Verysharp positive image, which was legible through the transparent support,was obtained.

EXAMPLE 5 The following hydrophobic composition:

Behenic acid, g 2.5

Carbon tetrachloride, ce. 97.5

Colloidal carbon having an average particle size of was coated twice ata rate of 20 sq. m. per lite on a porous paper base of 60 g. per sq. rn.provided with a gelatin interlayer coated from a solution of 50 g. ofgelatin in 940 cc. of water at a rate of 20 sq. m. per litre.

As schematically represented in FIG. 1 the obtained copying material 19consisting of the paper support 27 containing the hydrophobic meltablelayer 20, which contained the infrared and visible light-absorbingcarbon particles in such a concentration that the optical density of thelayer was 0.50, was pressed against the original 21 (which was atransparency) and a black paper 28 against a glass cylinder 24 having adiameter of 8 cm. In the axis of this cylinder a xenon gas dischargelamp 25, which on discharge between the electrodes 26 produced aradiation energy of 600 watt. sec. in 1/2000 sec., was placed at adistance of 4 cm. from the original 21 having visible light-absorbingimage-markings 22 and background areas 23, which are transparent tovisible light.

After exposure the layer 20 was brought in Contact with an aqueousdyestutf solution by means of a roller, the dyestuff solution containing2% by weight of a mixture of malachite green and fuchsin (S0/50). Thedye was absorbed in the areas of the copying material corresponding withthe transparent areas 23 of the original.

EXAMPLE 6` A subbed polyethylene terphthalate support was coated in aproportion of 50 g. per sq. In. with a layer from the followingsolution:

Gelatin, g 24 Sodium stearate, g. Water to cc. 1000 After drying, aheat-sensitive layer was applied thereto from a suspension of thefollowing composition, which has been ground for 8 hours in a ball mill:

Stearic acid, g. 200 Carbon black, g. 1.5 Carbon tetrachloride, cc 1000EXAMPLE 7 A cellulose triacetate film base was coated consecutively withthe compositions A and B.

Composition A, after drying, forming a hydrophilic layer of 10p:

Ethanol, cc. 50 n-Butanol, cc 50 Crystal violet, g. 5

A 10% solution of bleached shellac dissolved in ethanol, cc .f. 50

Composition B, after drying, forming a hydrophobic layer of 2p:

Montan wax, g. 10 White spirit, ce. 100

The recording layer was reectographically exposed with its hydrophobiclayer in contact with the infrared light-absorbing image markings of atext typed on a common paper sheet. After the exposure a hectographicprinting master ready for printing with the aid of ethanol as printingliquid was obtained.

EXAMPLE 8 A polyethylene terephthalate lm base was coated consecutivelywith the following compositions C and D.

Composition C, after drying, forming a hydrophilic layer of 10p.:

Ethanol, cc. 100 n-Butanol, cc. 50 Crystal violet, g. 5

Composition D, after drying, forming a hydrophobic layer of 2p:

Microcrystalline petroleum wax having a melting point of 76-79 C., soldunder the trade name Warco Wax 170A Yellow (a wax manufactured byWarwick Wax Co. Inc.-U.S.A.), g. 12.5

Carbon tetrachloride, cc. 100

The recording layer was reectographically exposed to infra-red lightwith its hydrophobic layer in contact with the infra-red light-absorbingimage markings of a text typed on a common paper sheet.

After exposure a hectographic printing master ready for use with ethanolas printing liquid was obtained.

EXAMPLE 9 A transparent glassine paper was coated pro rata of 80 g. persq. m. with the following composition:

Polyacrylamide, g. 50 Sodium thiosulphate, g. 50 Water, cc. 1000 On thedried hydrophilic layer of hydrophobic layer was applied from thefollowing composition:

Microcrystalline petroleum wax having a melting point of 76-79 C., soldunder the trade name Warco Wax 170A Yellow (a wax manufactured byWarwick Wax Co. Inc.-U.S.A.), g. 5

Carbon tetrachloride, cc.

The dried hydrophobic layer had a thickness of 5p.

The recording material was retlectographically exposed to infrad-redradiation with the hydrophobic surface layer in contact with theinfra-red absorbing image markings of a typed text.

After the exposure the surface layer was rubbed with a cotton swabsoaked with a aqueous solution of silver nitrate. A very dense black,sharpcopy which was legible through the transparent paper support wasobtained from the original.

What we claim is:

1. A heat-sensitive recording material comprising a support, awater-permeable hydrophilic colloid layer adjacent said support and anexternal water-impermeable hydrophobic layer in contact with saidcolloid layer, said hydrophobic layer consisting essentially of at leastone 'hydrophobic substance which melts on heating and in such moltencondition is compatible with said hydrophilic colloid layer.

2. A heat-sensitive recording material according to claim 1, whereinsaid substance is an organic substance melting between 40 and 200 C.

3. A heat-sensitive recording material according to claim 1 wherein saidsupport is formed of non-porous material.

4. A heat-sensitive recording material according to claim 1, wherein thehydrophobic layer is composed of or mainly contains fatty acids, themetal salts thereof, fatty alcohols, fatty ethers, fatty esters, fattyketones, parafnic compounds, or a mixture of such substances.

S. A heat-sensitive recording material according to claim 1, wherein thehydrophobic layer is composed of or mainly contains a natural wax, achemically modified natural wax, or a synthetic wax.

6. A heat-sensitive recording material according to claim 1, wherein thehydrophobic layer is composed of or consists mainly of an organicsublimable hydrophobic substance.

7. A heat-sensitive recording material according to claim 1, wherein thehydrophilic layer contains a `hydrophilic dye.

8. A heat-sensitive recording material according to claim 1, wherein thesupport is transparent.

9. A heat-sensitive recording material according to CTI claim 1, whereinat least one of the layers of the recording material contains a Ivisibleand infrared light-absorbing pigment that converts absorbed light intoheat.

10. A recording method, wherein said heat-sensitive material of claim 9is exposed to a pattern of said light.

11. A recording method according to claim 10, wherein said exposure isof high intensity and short duration.

12. A recording method, wherein said heat-sensitive recording materialof claim 1 is subjected to the action of information-wise modulated heatto form a latent image according to said information.

13. A recording method according to claim 12, Wherein said latent imageis developed with a liquid selectively wetting the heated portions ofthe surface layer.

14. A recording method according to claim 13, wherein said liquid is anaqueous liquid.

15. A recording method according to claim 14, wherein the aqueous liquidcontains a substance selected from the group consisting of a dye, acolour coupler, a catalyst for a colour reaction, and a bleaching agent.

16. A thermographic recording method according to claim 12, wherein thehydrophilic colloid layer contains a dyestuff suited for application inhydrotype or 'hectographic printing and the external hydrophobic layer,after having been image-wise subjected to the action of heat, ismoistened with a hectographic printing liquid and pressed against aprinting paper.

References Cited UNITED STATES PATENTS 3,156,183 11/1964 Bach lOl-4673,223,838 12/1965 Hoshino et al. Z50-65.1 3,283,708 ll/l966 YackelZ-65.1 X 3,298,833 1/1967 Gaynor 96-27 3,418,469 12/1968 Gold 250-65.1

RALPH G. NILSON, Primary Examiner A. L. BIRCH, Assistant Examiner U.S.Cl. X.R. lOl-467; 96-27

